1. Field of the Invention
Apparatuses and methods consistent with the present invention relate to optical fiber communications devices and, more particularly, to variable light controlling devices and variable light controlling methods for controlling optical characteristics variably on the basis of thermo-optic effects.
2. Description of the Related Art
In Wavelength Division Multiplex (WDM) optical communications, the need for a technique for tuning the wavelength of light signals is becoming important as Add Drop Multiplex (ADM) and other functions are enhanced more and more. At present, to realize this technique, wavelength variable devices are employed. As an example of one of such devices, there is provided a wavelength tunable filter for transmitting or blocking optical signals having a particular wavelength. In the related art, as shown in Japanese Patent Laid-Open Application No. 1988-281104, and Japanese Patent Laid-Open Application No. 1987-100706, a thermo-optic effect type filter is proposed. These thermo-optic effect type filters include resonators formed on a substrate, by using a silica waveguide processing technique. And, as shown in PCT application WO 2005/096462, there is another related art device which is a wavelength tunable laser for outputting optical signals having a particular wavelength. PCT application WO 2005/096462 shows a device in which an external resonator is composed by a thermo-optic effect filter type resonator and a semiconductor optical amplifier (SOA).
In the case of the related art wavelength variable devices, as described above, plural elements as well as resonators can be manufactured together in a silica waveguide process. Therefore, the characteristics of such related art devices are determined by an accuracy of masks and processing that are employed. The characteristics of the devices which are manufactured by optical waveguide processes which adopt an accurate mask and accurate processing are stabilized and desired characteristics are obtained easily. Because such devices have excellent characteristics, such devices are expected to be developed more and more in the future.
In any of the above related art devices, tuning wavelength is realized by heating a waveguide, which has thermo-optic effects, with a heater to change the temperature of the waveguide. However, when heating the waveguide, the following problems arise. For example, when the heater is powered to heat the waveguide, the temperature of the substrate is also changed at the same time. Generally, the temperature of the substrate is controlled to be constant by using a thermistor and a peltier element to stabilize the temperature of other elements. Therefore, when the temperature of the substrate is changed due to the heater, which is used to change the temperature of the waveguide, the thermistor detects the temperature change and the peltier element is driven to stabilize the temperature of the substrate. This series of operations by the thermistor and the peltier element generally takes about 10 seconds. And, the wavelength is not stabilized until the temperature of the substrate is stabilized. Therefore, the related art devices discussed above need extra time to vary and stabilize the wavelength. Thus, all of the related art devices discussed above have a problem in that they cannot perform quick wavelength tuning operations (quick responses to wavelength tuning) within a time frame of 0.1 seconds to one second, which is needed for ADM functions, etc.